What international law are you referring to? I think a better question might be whether the UK government actions contravene its own European Union (Withdrawal) Act 2018.
– andrew.punnettMar 28 '19 at 2:11

After reading the act, I believe that the European Communities Act 1972 will be withdrawn on 29 March unless a majority in parliament votes to amend the European Union (Withdrawal) Act 2018. However, on 29 March almost all UK law which relates to the EU will remain in place, so this will not have a huge affect provided that the nations in the EU amend their laws to allow UK citizens and companies the same benefits they currently receive.
– andrew.punnettMar 28 '19 at 2:23

In fact the main changes to UK law that will occur due to the European Union (Withdrawl) Act 2018 are that it gives ministers the ability to dictate legislation without the permission of parliament or the people (Section 8) and it allows ministers to enact rules that violate citizens fundamental rights (Section 5).
– andrew.punnettMar 28 '19 at 2:31

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Actually section 20(4) of the act allows government ministers the ability to 'amend the definition of “exit day”', so I assume that is what they plan to do.
– andrew.punnettMar 28 '19 at 2:42

2 Answers
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Treaties have to have enabling legislation in the United Kingdom. They do not have direct force of law by themselves. There is no written constitution, and the purported hierarchy of laws given in another answer is incorrect. In fact, U.K. courts do not even usually consult the texts of the treaties to determine what the law is; it is firstly and foremostly taken to be whatever the enabling legislation says, with treaties and other documents only consulted if the enabling legislation is ambiguous or open-ended.

The European Communities Act 1972 was one such piece of enabling legislation. You can see the specifics of enabling the treaties in its § 2. The European Union (Withdrawal) Act 2018 repeals it.

The latter Act specifies that this happens on what it terms "exit day". Like many Acts, it has an "interpretation" section for its jargon terminology, in this case § 20. That section defines "exit day" as 2019-03-29. But it also provides a mechanism for "exit day" to be altered by Statutory Instrument, if the treaty processes end up with a different day actually being exit day. The normal procedure for SIs is for a Minister of the Crown to lay them before Parliament, and Parliament approves or rejects them. If approved, they become law.

And that is what has happened. The SI amending exit day went before Parliament yesterday evening at 21:00 GMT. It was approved, by a majority of 441 votes to 105 in the House of Commons at 21:19 GMT. (At the time of originally writing this answer, the WWW site publishing the SI had not been updated to reflect its approved status. Technically, both houses of Parliament must approve it, and I was still waiting on the House of Lords. I noted that was extremely unlikely to be blocked in the Lords, and most people are regarding it as de facto approved.) It was approved in the House of Lords the same evening, and is now listed as made at 12:40 GMT today, 2019-03-28.

Gordon Anthony (2002). UK Public Law and European Law. Hart Publishing. ISBN 9781841131481.

Monica Claes (2016). "The primacy of EU Law in European and national law". In Anthony Arnull and Damian Chalmers: The Oxford Handbook of European Union Law. Oxford University Press. ISBN 9780199672646.

Your answer is more detailed and IMHO better, but I respectfully disagree with you characterizing mine as incorrect. At best it's incomplete. As you point out, the UK overcomes its lack of a Constitution by passing legislation to make treaties come into force. Other countries would simply ratify the treaty, at times alongside tweaks to the Constitution (e.g. France for the Lisbon Treaty). Also, best I can recollect, the ECJ view is that EU law trumps national law when an ambiguity between the two is brought to its attention. So at least for EU law the traditional hierarchy holds in the UK too.
– Denis de BernardyMar 28 '19 at 10:20

No, it is quite right to characterize that as incorrect. I already pointed to a book on this subject. Pay particular attention to chapter 4.
– JdeBPMar 28 '19 at 10:42

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"Treaties have to have enabling legislation in the United Kingdom." This is not true in all cases. From this Commons briefing paper: "the Government must lay most treaties before Parliament for 21 sitting days before it can ratify them, and the Commons can block ratification indefinitely. However, there is no statutory requirement for a debate or vote, and parliament cannot amend treaties." However, there are exceptions, including EU treaties, which need an enabling Act.
– Steve MelnikoffMar 28 '19 at 10:46

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You aren't understanding what you are reading. Treaty ratification is not the same as giving a treaty direct force of law.
– JdeBPMar 28 '19 at 12:27

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You are still not understanding what you are reading. The answer is in the context of the question, requires one to read more than individual sentences in isolation for reading comprehension, and does not say "all" in the first place.
– JdeBPMar 28 '19 at 17:36

The UK is a little different, since it has no Constitution or Founding Document. JdeBP's excellent answer has the procedural details on how this works in the UK.

In the specific case of the EU's body of treaties and laws, the UK position was clarified during the legal saga of Factortame Ltd versus the U.K. transport secretary, which ran from 1989 to 2000 over fishing rights. It saw the House of Lords confirm that EU law was supreme over British law in areas where there are competencies in the EU treaty.

Re the last case; this is due to EU law becoming, by British law, British law rather than EU law per se overriding it
– user19831Mar 28 '19 at 19:42

@Orangesandlemons: The UK argued that British immigration law applied therefor he should no longer be allowed to stay. Singh brought the case to the ECJ, which ultimately ruled that no, actually, EU law overrides British law in his case.
– Denis de BernardyMar 28 '19 at 19:47

It ruled that British law incorporated Eu law in a manner that overrode other British law. In fact the mechanism that makes EU law British law was a key factor in the courtcase which forced a vote on invoking article 50. Also worth noting that ECJ rulings will often be from a European law pov
– user19831Mar 28 '19 at 20:15